1. Field of the Invention
The present invention generally relates to optical limiters and more specifically, to performance enhancement systems and techniques for photorefractive crystal optical limiters.
2. Description of Prior Art
It is well known that optical sensors can be damaged by exposure to intense laser radiation of both pulsed and continuous wave (cw) varieties. Protectors or optical limiters may be of two general types, active or passive. Active optical limiters require a predetermination of the presence of threat radiation and then must provide an external stimulus (such as an applied electric field) to operate the limiter. Active devices are usually complex (tunable filters, for example) and are unable to rapidly respond to short pulses or bursts of harmful radiation. Passive protectors are preferred since the threat radiation itself triggers the desired protective response.
One of the more difficult aspects of sensor protection arises from the broad temporal range of available threats (i.e., lasers operating in temporal modes from continuous wave to picosecond pulses can pose serious problems for sensitive detectors). There are very few optical nonlinearities that can be used to passively protect a sensor over this broad range of temporal response. The photorefractive nonlinearity found in photorefractive crystals is one exception.
An example of photorefractive crystal power limiters may be found in U.S. Pat. No. 5,073,705 to Sharp et al. entitled "Broadband, Multi-line, Optical Power Limiting Scheme" issued Dec. 17, 1991, incorporated herein by reference. Photorefractive crystals as power limiters continue to have performance problems with regards to time response and beam depletion. The time response for photorefractive beam fanning depends inversely on the incident intensity which determines the limiting threshold. Currently, the time response in these types of limiters have yet to be optimized so as to decrease the limiting threshold. If ample time is allowed for the limiting to occur, even very weak cw beams (approx. 1 milliwatt) can be limited. Also, there is yet to be determined a way to maintain the maximum interaction length in the photorefractive medium while fanning the incident light, so that the maximum coupling of energy into the fanned beams and out of the incident beams (beam depletion) can be achieved.
While the prior art has reported using optical limiters based on photorefractive limiters none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a beam fanning limiter system and technique which achieves improved time response and maximum interaction length. This would yield truly passive broadband sensor protection against high intensity, short-pulse, high repetition rate multi-line lasers and multi-line cw lasers.